The present invention relates to an image forming apparatus and image forming method for forming images on both sides of a paper sheet in a printer unit on the basis of image data acquired by reading originals in a scanner unit.
A program scheme of a conventional image forming apparatus comprises a display control of a control panel, an input control in which the control panel is manually operated and print data is acquired from input data, and a system control section (hereinafter referred to as “system”) such as a print control for printing input data.
In a double-side printing operation, the print control first effects printing on a second side of a sheet and stacks it on an automatic double-side unit (ADU). The sheet size at the time of printing is determined by the input control when an original is read. The print control effects printing on a paper sheet which is fed from a sheet cassette in which paper sheets having the sheet size determined by the input control are set.
For printing on a first side, the sheet is fed from the automatic double-side unit. After printing, the sheet is discharged to a tray or a finisher. The sheet size for the first side is the same as the sheet size for the second side on which printing has already been effected.
However, in a multiple-size double-side printing in an automatic paper selector (APS) mode, the sheet size determined by the input control at the time of reading may differ between the second side and the first side, depending on the order of originals. In this case, the following problems will arise:
(1) Where the sheet size for the second side is smaller than the sheet size for the first side, omission of image will occur on the first side.
(2) Where the sheet size for the second side is greater than the sheet size for the first side, a blank portion will form on the first side.